The present invention relates generally to a fuel control system for an internal combustion engine and more particularly a fuel control system for an internal combustion engine of the type used in conjunction with an intake-air flow-rate measuring device disclosed in copending and commonly assigned application Ser. No. 665,443, filed Mar. 9, 1976, now U.S. Pat. No. 4,058,100, so that the fuel may be metered in quantity substantially proportional to the flow rate of intake air flowing through an intake tube in order to maintain a constant air-fuel ratio over the whole operating range of the engine.
There have been devised and demonstrated various types of fuel injection systems of the type which meter the fuel in quantity proportional to the flow rate of intake air flowing through an intake tube. In one type as disclosed in U.S. Pat. No. 3,927,649, there is provided a fuel metering piston which is shifted together with an intake air-measuring device so that the fuel may be delivered in quantity always proportional to the flow rate of intake air and consequently an air-fuel mixture with a uniform air-fuel ratio may be charged in cylinders of the engine. The fuel injection system of the above type has been further improved in order to more precisely control the air-fuel ratio of the air-fuel mixture. That is, in response to the output signal from an exhaust gas measuring probe mounted in an exhaust pipe for detecting the air-fuel ratio based upon the composition of the exhaust gases, the volume of air bypassing the air-measuring device is so controlled that the air-fuel mixture with a uniform air-fuel ratio may be always delivered to cylinders of the engine.
In the above fuel injection system in which the air-fuel ratio of the air-fuel mixture is controlled by controlling the volume of air bypassing the air-measuring device, the volume of intake air measured by the air-measuring device is less than the whole volume of intake air supplied to the engine cylinders by the volume of air bypassing it so that the measurement of intake air is deviated from a correct value and thus unsatisfactory metering of fuel results. Furthermore the response characteristics of the air-measuring device are slow, thereby resulting in slow response of fuel metering. These drawbacks are amplified especially when the volume of intake air is little. For this reason, the system of the above type has been unsatisfactory to maintain a constant air-fuel ratio.